Optimizing the Scavenging System for a Two-Stroke Cycle, Free Piston Engine for High Efficiency and Low Emissions: A Computational Approach

A free piston internal combustion (IC) engine operating on high compression ratio (CR) homogeneous charge compression ignition (HCCI) combustion is being developed by Sandia National Laboratories to significantly improve the thermal efficiency and exhaust emissions relative to conventional crankshaft-driven SI and Diesel engines. A two-stroke scavenging process recharges the engine and is key to realizing the efficiency and emissions potential of the device. To ensure that the engine’s performance goals can be achieved the scavenging system was configured using computational fluid dynamics (CFD), zero- and onedimensional modeling, and single step parametric variations. A wide range of design options was investigated including the use of loop, hybrid-loop and uniflow scavenging methods, different charge delivery options, and various operating schemes. Parameters such as the intake/exhaust port arrangement, valve lift/timing, charging pressure and piston frequency were varied. Operating schemes including a standard uniflow configuration, a low charging pressure option, a stratified scavenging geometry, and an over-expanded (Atkinson) cycle were studied. The computational results indicated that a stratified scavenging scheme employing a uniflow geometry, and supplied by a stable, low temperature/pressure charge will best optimize the efficiency and emissions characteristics of the engine. The operating CR can be maximized through substantial replacement of the burned charge, while short-circuiting emissions can be controlled by late fuel introduction. The in-cylinder flows are important to both NOx and short-circuiting emissions with inadequate mixing (and resulting temperature stratification) the predominant driver of NO production, and fuel penetration to the exhaust valve region the main cause of unburned hydrocarbon emissions.

[1]  R. Reitz,et al.  Modeling and Experiments of HCCI Engine Combustion Using Detailed Chemical Kinetics with Multidimensional CFD , 2001 .

[2]  Wolfgang Zahn,et al.  Analysis of a Stratified Charging Concept for High-Performance Two-Stroke Engines , 2000 .

[3]  C. Stan,et al.  Development of a Direct Injection Concept for Two Wheelers Equipped with Two Stroke Engines , 1999 .

[4]  Robert Morgan,et al.  Air-fuel mixing in a homogeneous charge DI gasoline engine , 2001 .

[5]  R. R. Maly,et al.  Evolution-Strategy Based, Fully Automatic, Numerical Optimization of Gas-Exchange Systems for IC Engines , 2001 .

[6]  Seiji Hirano,et al.  EMISSION CONTROL OF TWO-STROKE MOTORCYCLE ENGINES BY THE BUTTERFLY EXHAUST VALVE , 1980 .

[7]  Constantin Pana,et al.  Variable Valve - Control Systems for Spark Ignition Engine , 2001 .

[8]  Koji Morikawa,et al.  Mixture Formation Analysis of a Schnurle-Type Two-Stroke Gasoline DI Engine , 2001 .

[9]  Mariano Migliaccio,et al.  Modeling the Mixture Formation in a Small Direct-Injected Two-Stroke Spark-Ignition Engine , 1997 .

[10]  Robert Hoekstra,et al.  Optimizing Internal Combustion Engine Performance Through Response Surface Methodology , 1996 .

[11]  Robert Kee,et al.  THE REDUCTION OF EXHAUST EMISSIONS FROM TWO-STROKE ENGINES , 1993 .

[12]  Daniel S. Sanborn,et al.  Single Cycle Simulation Simplifies Scavenging Study , 1985 .

[13]  Mukesh Saxena,et al.  A Stratified Charging Two-Stroke Engine for Reduction of Scavenged-Through Losses , 1989 .

[14]  Rolf D. Reitz,et al.  Diesel Engine Combustion Chamber Geometry Optimization Using Genetic Algorithms and Multi-Dimensional Spray and Combustion Modeling , 2001 .

[15]  J. M. Novak,et al.  Computer Optimization of Camshaft Lift Profiles for a NASCAR V-8 Engine with Restrictor Plate , 1996 .

[16]  Michael Bargende,et al.  Direct Coupled 1D/3D-CFD-Computation (GT-Power/Star-CD) of the Flow in the Switch-Over Intake System of an 8-Cylinder SI Engine with External Exhaust Gas Recirculation , 2002 .

[17]  Gordon P. Blair.  The Correlation of Theory and Experiment for Scavenging Flow in Two-Stroke Cycle Engines , 1988 .

[18]  Peter Van Blarigan,et al.  Homogeneous Charge Compression Ignition with a Free Piston: A New Approach to Ideal Otto Cycle Performance , 1998 .

[19]  Peter Van Blarigan,et al.  A numerical study of a free piston ic engine operating on homogeneous charge compression ignition combustion , 1999 .

[20]  Eran Sher,et al.  Minimizing Short-Circuiting Losses in 2-S Engines by Throttling the Exhaust Pipe , 1990 .

[21]  S. H. Raynes.  An Atkinson cycle engine for low pollution , 1998 .

[22]  Seiji Hirano,et al.  Improvement of Fuel Consumption with Variable Exhaust Port Timing in a Two-Stroke Gasoline Engine , 1985 .

[23]  Yoichi Ishibashi,et al.  A Low Pressure Pneumatic Direct Injection Two-Stroke Engine by Activated Radical Combustion Concept , 1998 .